1. Field of the Invention
The present invention relates to a resistance measurement method for measuring the electric resistance of a measurement object with a film formed on a surface thereof and a component inspection process to which the resistance measurement method is applied.
2. Description of the Related Art
Metal plate materials have been conventionally used as a material of cabinets for personal computers and racks for housing the personal computers. Such metal plate materials are inexpensive and easily available while providing certain strengths.
FIG. 1 is a view showing an external appearance of a rack in which an information processing unit such as a personal computer is housed.
Metal plates are used for a rack 3, a housing section 30 and a door section 31 shown in FIG. 1. When the metal plates are used for the rack 3, the housing section 30 and the door section 31 as in this case, the metal plates also serve as shield members to prevent unwanted radiation leak from the personal computer housed in the rack 3 to the outside.
However, the rack 3 tends to have a gap between the door and a section for rotatably supporting the door, or between the door and other sections. When there is unwanted radiation from the personal computer in the rack with such a gap, the radiation leaks to the outside of the personal computer through the gap. Thus, an unwanted radiation prevention member 32 made of a sheet-like conductive rubber, a conductive cloth or the like is provided in the gap to produce a better shield effect. In other words, a member such as the unwanted radiation prevention member 32 also serves as a shield member.
Recent personal computers tend to increasingly cause unwanted radiation at high frequency due to use of high frequency clocks for determining operation speeds. For this reason, in order to improve a shielding function, metal plate materials (or, organic materials having electric conductivity may possibly be used when the organic materials are less expensive) is used for a personal computer cabinet and for the housing section 30 and the door section 31 of the rack shown in FIG. 1.
When the cabinet of a personal computer, the rack, and the like are designed to have a shielding function, it is necessary to evaluate, in advance, the shielding performances of materials for cabinets and racks, such as a metal plate material, a conductive cloth and the like.
In the general evaluation of the shielding performance of a metal plate material, the electric resistance of the metal plate material is measured using a resistance measurement method in accordance with the specification of JIS-C-2550, and then the evaluation is made according to the principle that the lower the measured electric resistance, the higher the shielding performance. Currently, general-purpose galvanized steel plates or the like, which are easily available as described above, are used as plate materials for fabricating cabinets and racks for personal computers. The galvanized steel plates are surface-treated (anti-corrosive treated or the like) in a way different depending on a manufacturer. In some cases, the conditions of surface treatment may differ even by the single manufacturer. Therefore, when the evaluation is performed, the electric resistance of a plate including the surface treatment must be measured first, and then the characteristics including the surface treatment must be evaluated for the plate.
However, when the electric resistance of a metal plate material is measured by using the resistance measurement method in accordance with the specification of JIS-C-2550, a sharp probe breaks a film formed on the surface of plate, resulting in that the electric resistance of the steel plate under the film is measured. Consequently, the same values of the electric resistance are measured even though the steel plates have different surface treatments.
Hence, to solve this problem, a technique is proposed in Japanese Patent Application Publication No. Sho 57-154069, in which a conductive rubber is used to measure the electric resistance without breaking a film on the surface. The conductive rubber used in Japanese Patent Application Publication No. Sho 57-154069 is a conductive material having almost the same characteristics as metal. The conductive rubber is used as a material preventing unwanted radiation by filling a gap between the housing section and the door section as shown in FIG. 1. The conductive rubber is also used as a member to easily connect a land pattern and a terminal of a leadless part on a substrate as proposed in Japanese Patent 4 Application Publication No. Sho 56-79262. The technique described in Japanese Patent Application Publication No. Sho 57-154069 enables the performance evaluation of each of plate materials manufactured by different manufacturers, and also enables the performance evaluation of each of multiple plate materials manufactured by a single manufacturer. Accordingly, plate materials suitable for the fabrication of cabinets and racks can be appropriately selected.
Although the technique of Japanese Patent Application Publication No. Sho 57-154069 enables measurement of a resistance value of a plate material, it is difficult to apply the technique to measurement and evaluation of materials such as a conductive rubber and a conductive cloth because these materials tend to deform easily. In the case of the rubber or the conductive cloth, difficulty in maintaining its shape makes it difficult to obtain stable measured values. Therefore, a technique enabling accurate and stable measurement of resistance value has been desired. Further, in some cases, paints are applied to some procured plate materials before or after the plate materials are assembled into cabinets and racks. In such a case, since a film formed by the application of the paint affects the shielding characteristics of the cabinets and the like, a technique enabling accurate and stable measurements of resistance values has also been desired for the rubber and the cloth.